1. Field of the Invention
The present invention relates to miniaturizing the entirety of a radio circuit apparatus such as a cellular phone utilized in a high frequency radio communication system which employs the time division multiplex system.
2. Related Art of the Invention
In recent years, along with the development of mobile communication related markets such as that of cellular phones, a variety of application products utilizing a radio circuit apparatus appear in the market, and the miniaturization of radio circuit apparatus is becoming more and more important.
FIG. 5 shows an example of the structure of a conventional radio circuit apparatus. In this figure, 401 is a reference oscillator, 402 is a first local oscillator, 403 is a second local oscillator, 404 is a transmitting circuit, 405 is a receiving circuit, 406 is a shared device and 407 is an antenna. The output signal of the reference oscillator 401 is inputted to the first local oscillator 402 and the second local oscillator 403. And the output frequency of the second local oscillator 403 is fixed when the radio circuit apparatus is transmitting and when it is receiving, respectively.
In the transmitting circuit 404, 411 is a modulator, 412 is a filter, 413 is a transmitted frequency converter, 414 is a filter and 415 is a power amplifier. When the radio circuit apparatus is transmitting, the modulator 411 modulates the output signal of the second local oscillator 403 with a base band signal (a modulated signal).
The output signal of the modulator 411 is inputted into the transmitted frequency converter 413 after being reduced in the unnecessary frequency components by the filter 412, and converted into a frequency of a transmitted signal by the output signal of the first local oscillator 402. The output signal of the transmitted frequency converter 413 is amplified at the power amplifier 415 after being reduced in the unnecessary components by the filter 414, and transmitted from the antenna 407 through the shared device 406.
In the receiving circuit 405, 421 is a low noise amplifier, 422 is a first received frequency converter, 423 is a filter and 424 is a second received frequency converter. A signal received on the antenna 407 is inputted into the low noise amplifier 421 through the shared device 406. The output signal of the low noise amplifier 421 is inputted into the first frequency converter 422, and converted to the first intermediate frequency by the output signal of the first local oscillator 402. The output signal of the first frequency converter 422 is reduced in the unnecessary frequency components by the filter 423. The output signal of the filter 423 is inputted into the second frequency converter circuit 424 to be converted to the second intermediate frequency by the output signal of the second local oscillator 403.
In the above mentioned structure, however, when the radio circuit apparatus is transmitting, a harmonic component of the output signal from the second local oscillator 403 is generated in the transmitted frequency band, and the harmonic component tend to become an unnecessary frequency component for cellular phones close by. To prevent this problem, a large number of filters need to be provided, and as a result, the radio circuit apparatus as a whole would be bulky.